Image capturing apparatuses which can record captured images as digital data, such as digital still cameras and digital video cameras, have been widely spread in recent years. Demands for more compact and more inexpensive digital image capturing apparatuses with a higher-magnification zooming function have been increased. It is known that lenses for the image capturing apparatuses cause optical image distortion due to so-called distortion aberration especially at the wide-angle side. Such distortion tends to become larger when the optical system becomes shorter or the zooming function provide a higher magnification. To manufacture lenses having low distortion, the cost increases.
Such image distortion has been corrected by applying image processing to an image signal obtained from image capturing. It is known that the amount of distortion aberration is proportional to the cube of the distance from the center of the light receiving plane of the image capturing device. A method has been generally used in which the coordinate system of a before-correction image is converted to a coordinate system formed of the image center and the distance therefrom, the positions of the original pixels are shifted by the amount of aberration according to the above-described characteristics, and then the coordinate system is converted to the coordinate system of a corrected image according to the data of the pixels.
However, such a theoretical calculation method needs to frequently use floating-point arithmetic calculations and trigonometric functions, requiring a vast amount of calculation time and high calculation capacity. In addition, spherical lenses have been used increasingly these days, and it is difficult to obtain their aberration characteristics by calculations in a simple manner. In contrast, a method has been proposed in which the after-conversion coordinates depending on the amount of aberration, of each pixel in an image are stored in advance in a read only memory (ROM), and image data after coordinate conversion is written in an image memory, based on the stored information (for example, see Japanese Laid-open Utility Model Application Publication No. 05-48051 (paragraph numbers [0012] to [0013], FIG. 1).
As a conventional technique related to image distortion correction, a distortion correction method has been used, in which, to correct image distortion caused by the projection direction when an image is projected on a screen by a projector, the original image is attached to a polygonal mesh indicating the shape of the screen as texture by using condition parameters at the observer side, the polygonal mesh is drawn by using projection condition parameters at the projector side, and the drawn image is projected (for example, see Japanese Laid-open Patent Publication No. 2004-72553 (paragraph number [0005], FIG. 1).
As another related technique, a digital copying machine has been used in which, to copy an opened book, correction is applied to the read image such that an area where the backbone of the book is located, which has distortion, is enlarged in the sub-scanning direction (for example, see Japanese Laid-open Patent Publication No. 2001-16428 (paragraph numbers [0074] to [0077], FIG. 6).
As described above, to use theoretical calculations to correct image distortion caused by distortion aberration, a vast amount of calculations are necessary. Therefore, when hardware is used to perform the calculations, the circuit scale becomes large. Especially when an spherical lens is used, it is almost impossible to perform the calculations by hardware. When software is used to perform the calculations, a very high calculation capacity is necessary. Therefore, it is difficult to implement such distortion correction calculations in image capturing apparatuses.
As the technique disclosed in Japanese Laid-open Utility Model Application Publication No. 05-48051 (paragraph numbers [0012] to [0013], FIG. 1), when a method is used in which the coordinates of each pixel after conversion are read from the ROM and mapped with the coordinate system of a before-correction image being used as a reference, after coordinate conversion processing is performed in the scanning order of the before-correction image, data calculation processing needs to be executed for each pixel in the scanning order of the corrected image. Therefore, processing efficiency is low and high speed processing is difficult. In addition, if rounding off is used to calculate each pixel of the corrected image after mapping, an area having no data is generated in the image. Therefore, the image correction precision becomes low, and extra calculations are necessary to fill the area.
As the technique disclosed in Japanese Laid-open Utility Model Application Publication No. 05-48051 (paragraph numbers [0012] to [0013], FIG. 1), when the after-conversion coordinates depending on the amount of aberration are stored in advance in a ROM for each pixel, the capacity of the ROM needs to be large, increasing the circuit scale and manufacturing cost.